1. Field of the Invention
The present invention relates to a process for producing an ethylene/.alpha.-olefin copolymer with an olefin polymerization catalyst constituted of a metallocene compound, an organoaluminum compound, and an ionizable ionic compound.
2. Description of the Related Art
The low-pressure Ziegler process for polymerization of ethylene or an .alpha.-olefin is well known in the related technical fields. The catalyst for the process is generally prepared by treating a mixture of an organometallic compound or hydride of a metal of Group 1A to 3A of Periodic Table with a compound of a transition metal (Group 3B to 2B of Periodic Table) in a suspension or a solution, or in the absence of a solvent or a diluent.
In recent years, special kinds of catalysts are being developed which are active in olefin polymerization. Examples of the catalyst are combination of a cyclopentadienyl derivative of a metal such as titanium, zirconium, and hafnium (Group 4B of Periodic Table) with aluminoxane. (See, for example, J. Boor: "Ziegler-Natta Catalyst and Polymerization", Academic Press, New York (1979), and H. Sinn and W. Kaminsky: Adv. Organomet. Chem. 1899 (1980).) These catalysts have ability of forming a stereospecific olefin polymer with high catalyst activity.
Nevertheless, such catalysts have not been used industrially mainly because of the following reasons: the aluminoxane cannot readily be produced in a reproducible form, hindering preparation of the catalyst and the polymer with required reproducibility, and the expensive aluminoxane has to be used in a considerably high ratio to the transition metal compound to achieve sufficient activity.
To offset the above disadvantages, Japanese Patent Laid-Open No. 3-207704 discloses ionic metallocene compound prepared by reacting a metallocene with an ionizing ionic compound. PCT Patent Application WO 92/01723 discloses a process of .alpha.-olefin polymerization by use of a catalyst system prepared by reacting a halogenated metallocene with an organometallic compound and then bringing the reaction product into contact with an ionizing ionic compound. This catalyst system is advantageous in olefin polymerization. However, when ethylene and .alpha.-olefin are copolymerized by use of such a catalyst at a high temperature, the resulting copolymer has a low molecular weight disadvantageously.
Japanese Patent Application Laid-Open No. 60-217209 discloses production of a high-molecular-weight olefin polymer by use of a catalyst comprising a hafnium compound as the metallocene component. In copolymerization of ethylene with .alpha.-olefin, however, this catalyst is much less active than the catalyst having zirconium as the ligand, and is disadvantageous in production efficiency of the copolymer.
The inventors of the present invention made comprehensive studies to solve the above problems, and found that an ethylene/.alpha.-olefin copolymer is obtained with a high catalyst activity by copolymerizing ethylene with .alpha.-olefin by use of a specific ionic metallocene catalyst at a temperature of not lower than 120.degree. C. The present invention has been accomplished based on the above findings.